Some thoughts:
The answer is likely a combination of multiple efforts:
1) the upper limit on those who are going to overdose has been reached... Those who continue to use do so while knowing how to not die from it.
2) less young users are fueling the crisis. There is a drop in drug use amongst teenagers, especially opioids. Over time, less new users backfill the population of current users. Newer users are more vulnerable to overdose than established ones.
3) narcan training and availability -most current opioid users have Narcan and many have used it on someone (I have worked with opioid users for 15 years, and used to be a heroin addict myself).
4) widespread buprenorphine access similarly has a protective effect. This also can mean that people stay on some type of opioid without completely abstaining for longer, however, so it may be complicating the overall population a bit.
5) difficult to parse out supply side changes... There is instability with the cartels, and there may also be a level of expertise that has emerged in producing a product that is more well balanced and less unpredictable.
6) awareness to the risks of use, public programming, and outreach to drug users regarding services and safe places to access their needs.
7) Wide embracing of Kratom as an opioid alternative is potentially helping folks move away from using illicits for pain management or as a way to self-maintain.
8) Legal cannabis as an alternative for some folks who use it to manage pain - this includes stuff like CBD and some of the other cannabinoids that seem to be catching on as alternatives to opioids. I've noted a lot of older folks who have started using these things as a way to manage pain.
The response to the crisis has been multifactorial and it's unlikely that any one piece has been the main factor. I think that we are seeing a shift due to the aggregate, which has taken a few years to fully buffer and protect. The goal now is to reevaluate policy to make things more tolerable for both drug users and the communities they live in.